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Open the main valve on the Cl2 cylinder.
A red glow will be observed.
The red glow is caused by the formation and decay of singlet O2.
Cl2 + H2O2 --(OH-)--> O2 + 2 HCl (the oxygen formed here is singlet oxygen)
Singlet oxygen is an excited form of oxygen. Oxygen normally has two unpaired electrons in its outermost orbital. Upon receiving this exitation, the two electrons occupy a single molecular orbital. Two singlet oxygens go to the ground state by a two molecule, one photon transition. This is actually a form of radioactive decay, and it causes the red glow.
2 O2(1Dg) -> 2 O2(3Sg) + hv (630 nm)